Modern passenger aircraft fly at high altitudes where the outside atmosphere is relatively thin. The passenger compartments in such aircraft are normally pressurized to a comfortable level somewhere below the equivalent of 10,000 feet altitude pressure. When the passenger compartments are depressurized for any reason to a higher altitude pressure, oxygen in some form is usually provided for passenger survival and comfort.
A common type of oxygen system used in modern passenger aircraft is a continuous flow type system which provides a continuous flow of oxygen to the user. Continuous flow type systems have been used for many years and are simple systems of design, cost, weight, and maintenance. Such systems offer reasonable safety for brief periods up to an altitude pressure of approximately 40,000 feet (12,192 meters) and are generally regarded as adequate up to 25,000 feet (7,620 meters) altitude pressure for prolonged protection.
There are several types of continuous flow systems which provide varying degrees of oxygen economy. In some continuous flow systems a flexible reservoir is used to collect oxygen during the breathing cycle, primarily in the exhalation phase. In one of these systems oxygen is diluted with ambient air in a flexible rebreather bag. In another system oxygen is diluted with ambient air in the oxygen mask facepiece. In this latter system, which will be used for illustrative purposes herein, the sequence of inhalation of oxygen and dilution air from the phase dilution mask is usually controlled by the valving to the facepiece.
In the prior art illustrative system oxygen flows into and expands a flexible reservoir. Upon inhalation, the first gas received by the passenger is the oxygen stored in the reservoir and when this is substantially exhausted the balance of the gas supplied on inhalation is primarily ambient cabin air. This system permits a higher efficiency in body utilization of oxygen. The gas reaching the alveolar gas transfer areas of the lung is richer in oxygen. The gas in the trachea and other dead spaces of the lung is richer in dilution air.
When oxygen masks are supplied to passengers following a depressurization, there is sometimes confusion and anxiety on the part of the passengers concerning the availability of the oxygen from the oxygen supply apparatus.
In the illustrative system and in other systems which use a flexible reservoir for oxygen storage and a dilution feature for augmenting the oxygen with ambient air, it is sometimes not immediately apparent if a particular passenger is receiving oxygen to his dispensing device. This difficulty is encountered because the passenger can breathe ambient cabin air which may not contain supplemental oxygen.
In the prior art illustrative system the usual way of determining the flow of oxygen to the reservoir and mask facepiece is to close off the oxygen outlet from the reservoir to the mask facepiece and permit the flexible reservoir to fill. The swelling of the flexible reservoir indicates a positive flow of oxygen. Due to the low rate of oxygen flow in the present system, for example, 0.31 liters/minute (18 in..sup.3 /min.), and particularly at lower altitudes, a delay as long as 20 seconds may be encountered at certain pressure altitudes before a positive indication of oxygen flow into the reservoir can be detected. In large aircraft transporting as many as 300 passengers and in certain emergency situations in smaller aircraft, it is desirable to avoid as much delay as possible in the determination of oxygen flow to the passengers. The purpose of the present invention is to provide an immediate and continuous indication of oxygen flow to the oxygen dispensing device without disrupting the flow of oxygen to the passenger.